(1) Field of the Invention
The present invention relates to the field of rotary-wing aircraft, such as rotorcraft equipped with at least one blade-carrying rotor, and it relates more specifically to mechanisms that equip such rotors in order to put the blades under stress by twisting them in their general plane of extension. The present invention relates to such a twist mechanism that is incorporable into a blade and that uses an actuator for actuating a twist member that is in engagement with the blade, in particular at the free end thereof. The present invention also relates to the associated blade.
(2) Background Art
Rotary-wing aircraft use a rotor to procure at least lift if not also propulsion. The rotary wing is made up of a plurality of blades that are distributed radially and that are carried individually by the hub of the rotor for being driven in rotation. A blade of the main rotor of a rotorcraft exerts lift when it is driven in rotation in order to procure lift for the rotorcraft. Flight controls are used to issue commands for changing the aerodynamic characteristics of the blades in order to act on the behavior of the rotorcraft as it is flying, by causing the angle of inclination of their surfaces of extension to vary relatively to the relative wind.
A blade is an airfoil that is highly elongate. A blade root end for anchoring the blade is assigned to engaging it with the hub of the rotor, its opposite end being considered to be a free end. The blade extends along its span over its surface of extension in a longitudinal dimension considered along a longitudinal axis between its anchor end and its free end, and in a transverse dimension considered between a leading edge and a trailing edge, which edges are opposite from each other. In the transverse dimension of the blade, which dimension is, in general, perpendicular to the longitudinal axis, there is defined a chord relative to which a camber line is formed and there are defined relationships of thickness on either side of said camber line so as to form, respectively, the suction-side surface and the pressure-side surface of each profile of the blade. Using various flight controls of the rotorcraft, the aerodynamic characteristics of the blades are modified by varying their pitch angle about the longitudinal axis that is referred to as the “pitch axis” and that extends in their longitudinal dimension. A cyclic flight control serves to modify the angle of incidence of the blades cyclically so as to influence the aerodynamic incidence of the blades and therefore so as to influence the progress of the rotorcraft, according to roll movements and pitching movements. A flight control modifying the collective pitch of the blades makes it possible to vary the altitude at which the rotorcraft is progressing.
The speed at which the rotorcraft progresses depends on the dissymmetry of the speeds between respectively an advancing blade that progresses from the rear towards the front of the rotorcraft, and a retreating blade that progresses in the opposite direction from the front towards the rear of the rotorcraft. The flow of air along the profile of the blades is determined by the shapes of their profiles, and varies depending on the situations in which the blades are placed during their rotation as advancing blade or as retreating blade. Such a variation in speed of air flow is commonly compensated by means of varying the angle of incidence of the blades, but it is observed that, as from a given angle of incidence, the retreating blade is placed in a stall situation. The speed of air flow over the retreating blade can be very low or indeed negative in a zone close to the hub of the rotor. Airstream separation is induced at the leading edge or at the trailing edge, resulting in a sudden drop in the lift procured by the blade if such separation propagates over a zone of longitudinal extension of the blade that is significant relative to its length. In addition, the airstream separation generates a vortex that is a source of vibrations and of an increase in the drag coefficient of the profile of the blade.
In order to avoid such a blade stall phenomenon, it is known that it is possible to twist the surface of extension of the blade in its longitudinal dimension. The twist of the blade may be permanent by being designed into it. However, it has become apparent that actively twisting the blade while the aircraft is in flight improves the performance of the rotary wing. The active twisting of the blades may be adapted according to rotary wing drive conditions and of the progress conditions of the rotorcraft.
Causing the blade to twist induces a progressive variation of incidence of profiles of the blade, between its free end and its anchor end, thereby making it possible to adapt each profile of the blade locally to suit the air flow speed between its leading edge and its trailing edge. In one general organization of a twist mechanism for twisting the blade, a twist member is put into engagement with the blade between its ends, and more particularly at its free end. Moving the twist member causes the blade to be put under torsion stress from one of its ends to the other end, thereby causing it to twist. In order to actuate the twist member, the rotor is equipped with means for moving the twist member, which means include a power source for powering an actuator of the twist member. Control means regulate activation of the actuator, in particular according to flight parameters, by being used in compliance with predefined flight commands and/or selectively by the pilot by means of manual control members.
For example, in Document U.S. Pat. No. 5,505,589 (BERGEY KARL H), the twist member is arranged as a ballast that is carried by the blade at its leading edge. The ballast is mounted to move in translation along the blade between the ends thereof, the ballast being moved by the actuator thereby locally generating torsion stress on the blade, thereby causing it to twist. The magnitude of the twisting of the blade varies depending on the position of the ballast between its ends, which ballast is moved by the actuator according to the flight conditions and/or of flight procedures. The actuator is an electrically driven member that is connected to the ballast via a mechanical transmission mechanism, such as of the type having a wheel and a worm screw. The actuator and the transmission mechanism are housed inside the blade, the actuator being electrically powered from the on-board power supply network of the rotorcraft.
Other ways of actively twisting the blade are known, such as using a shape-deforming element that is in engagement with and/or that is incorporated in the skin of the blade that forms its outside wall and that defines its suction-side and pressure-side surfaces. For example, the actuator may be of the piezoelectric type for urging the element to deform, and it may be powered from the on-board network of the rotorcraft. The magnitude of the twisting of the blade varies depending on the stress applied to urge the element to deform, while being caused by the piezoelectric means being activated by the control means according to the flight conditions and/or of the flight procedures. For example, reference can be made to Document US 2007/205332 (ONERA) that describes a blade twist mechanism of that type.
It is also known that it is possible to mount a flap hinged to the free end of a blade, and to move it so as to tilt it in flight using control means that are activated by the pilot. For example, in EP 0 734 947 (INST ADV TECH HELICOPTER LTD), such a flap is used to procure incidence for a blade. The flap is movable by an actuator that is housed inside the blade and that is in communication with the flap via a transmission mechanism having hinged links. The actuator is an electric motor member that is powered from an on-board network of the rotorcraft.
A problem that arises lies in the ways of mounting, inside a blade, an active twist mechanism of the type suitable for putting the blade under torsion stress according to flight commands issued by the pilot. Account needs to be taken of the facts that the space available inside the blade is small and that incorporating the twist mechanism for twisting the blade should not give rise to imbalance or disturb the behavior of the blade when it is driven in rotation. In addition, the twist mechanism must be compact but nevertheless robust and reliable, and incorporating it into the internal volume of the blade must be made safe in view of the hostile environment in which it is placed. The twist mechanism is subjected to large amounts of vibration and of mechanical stress due to the rotary wing rotating, and the way it is mounted inside the blade must be adapted accordingly.
The actuators used to twist the blades are piezoelectric or electromechanical power members, use of which involves grouping together structural means that are complex and costly. Such structural means must procure reliable and safe operation of the actuators with regard to the electrical nature of their energy source and to the hostile environment in which they are mounted. The actuators are movement-producing means having developed power that must be adapted to suit moving the twist members, while being sufficient to deform the surface of extension of the blade, and finally the blade as a whole. The structure of the actuators is complex, it being made up of numerous heavy and/or voluminous members, such as coils, magnets, and pins. The electrical power supply for powering the actuators from the on-board network of the rotorcraft must be safe and reliable, and must deliver power adapted to use of the actuators. The electrical power is brought from the on-board network to the free end of the blade by means of voluminous cables of large section that extend inside the blade. Passing the cables between the axis of rotation of the rotor and the internal space in the blades involves installing rotary electrical connections that offer good performance and that are safe, which connections are costly. An actuator of the piezoelectric type is less demanding in terms of power to be delivered, but it involves using high-voltages which must be conveyed safely. The hostile environment of a moving blade is not favorable to the use of power cables carrying high voltages, and is a source of major difficulties and of considerable costs for organizing, making safe, and maintaining an electricity network that is suitable for powering the actuator and that is received inside the blade.
In a technique remote from twisting the blades of a rotary wing, it is also known that it is possible to use a flyweight to move flaps placed at the end of a blade. For example, Document GB 627 117 describes a device for managing a flow rate of air expelled at the end of a blade equipping a rotary wing of a rotorcraft. That device has nozzles that are equipped with closure flaps for regulating the flow rate of air that they expel. Those flaps are movable by a lever arm that is mounted to pivot at the end of the blade, and that carries a counterweight causing the lever arm to pivot in opposition to a return spring.
Documents DE 198 59 041 and EP 1 083 123 are also known.